Attitudes toward physically punishing misbehaving children have changed drastically over the last 20-plus years, according to a new study that will appear in the December issue of Pediatrics. Spanking is out and time-outs are in across all income levels in the U.S., according to the study.

The researchers looked at four national studies that were conducted between 1988 and 2011, all of which asked mothers of different economic backgrounds if they used physical punishment (as opposed to time-outs or talking to the child) on their five-year-olds. (Kindergarten-age is one of the most common time to spank, according to the study.) In 1988, 46 percent of mothers at the 50th income-percentile endorsed physical punishment. By 2011, that number had dropped to 21 percent. At the 10th income-percentile, mothers endorsing time-outs increased from 51 percent to 71 percent over the same time period.

Mothers in this percentile have been getting closer to the numbers shown by their wealthier peers. Those in the 90th income-percentile were less likely to spank even back in 1988. But since the American Academy of Pediatrics issued its policy statement against physical punishment in 1998, the difference in punishment styles between these two groups has shrunk dramatically. “The 90/10 income gap decreased from 23 to 14 percentage points between 1998 and 2011,” according to the report.

“The current study suggests that these cultural shifts reflect real changes in the way parents discipline children,” the authors wrote. “Our results also show changes in discipline strategies before and after 1998, suggesting the AAP statement reflected and possibly catalyzed these shifts.”

Emily Landes has a six-year-old, a toddler and a pretty severe sleep deficit.